Conventionally, there is known a thread cutting device of a sewing machine which cuts a thread at an end of a sewing work (see, e.g., Japanese Patent No. 3106472).
As shown in FIG. 14A, a thread cutting device is arranged inside a bed portion of a sewing machine, and includes a thread catching member 610 which receives a power from a lower shaft interlocking with a sewing machine motor. The thread catching member 610 moves back and forth, and catches a thread T with a catching portion on a tip of the thread catching member 610 during a backward movement. Thereafter, the thread T is cut by the thread catching member 610 and a fixed knife.
Recently, there has been developed a thread cutting device of a sewing machine which employs a stepping motor for a driving source of a thread catching member, thereby driving the thread catching member at a low speed.
However, in the thread cutting device disclosed in the Japanese Patent No. 3106472, there is a limit in lowering a rotation speed of a servo motor employed as a sewing machine motor. More specifically, it is difficult to control the rotation speed of the servo motor in a low speed region. Therefore, it is difficult to sufficiently lower an operating speed of the thread catching member 610.
Accordingly, a bobbin 320 is rotated excessively when the lower thread T is vigorously pulled out from the bobbin 320 after being caught. As a result, as shown in FIG. 14B, there is a problem that the lower thread T is excessively pulled out from the bobbin 320. It can be proposed to suppress the rotation of the bobbin 320 by, for example, a biasing force of a spring. However, in a sewing machine which carries out a sewing work with various threads having different thickness and hardness, it is difficult to regulate such a biasing force appropriately. As a result, in a case in which the biasing force is too strong, there is a problem that the lower thread T might become entangled in the bobbin 320 so that it is difficult to pull out the lower thread T in a next sewing work.
In a case in which the thread catching member 610 is driven by a stepping motor, which is suitable for a low speed rotation and an angle control, a large and expensive stepping motor is required to ensure a sufficient torque for cutting a thick thread, for example. When an output torque of the stepping motor is insufficient for cutting a thick thread, there is a problem that a cutting failure is caused.